[Intel-gfx] [PATCH] drm/i915 : Avoid superfluous invalidation of CPU cache lines

[akash.goel at intel.com]

From: Akash Goel <akash.goel at intel.com>

When the object is moved out of CPU read domain, the cachelines
are not invalidated immediately. The invalidation is deferred till
next time the object is brought back into CPU read domain.
But the invalidation is done unconditionally, i.e. even for the case
where the cachelines were flushed previously, when the object moved out
of CPU write domain. This is avoidable and would lead to some optimization.
Though this is not a hypothetical case, but is unlikely to occur often.
The aim is to detect changes to the backing storage whilst the
data is potentially in the CPU cache, and only clflush in those case.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Signed-off-by: Akash Goel <akash.goel at intel.com>
---
 drivers/gpu/drm/i915/i915_drv.h | 1 +
 drivers/gpu/drm/i915/i915_gem.c | 9 ++++++++-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index df9316f..fedb71d 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -2098,6 +2098,7 @@ struct drm_i915_gem_object {
 	unsigned long gt_ro:1;
 	unsigned int cache_level:3;
 	unsigned int cache_dirty:1;
+	unsigned int cache_clean:1;
 
 	unsigned int frontbuffer_bits:INTEL_FRONTBUFFER_BITS;
 
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 19c282b..a13ffd4 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -3552,6 +3552,7 @@ i915_gem_clflush_object(struct drm_i915_gem_object *obj,
 	trace_i915_gem_object_clflush(obj);
 	drm_clflush_sg(obj->pages);
 	obj->cache_dirty = false;
+	obj->cache_clean = true;
 
 	return true;
 }
@@ -3982,7 +3983,13 @@ i915_gem_object_set_to_cpu_domain(struct drm_i915_gem_object *obj, bool write)
 
 	/* Flush the CPU cache if it's still invalid. */
 	if ((obj->base.read_domains & I915_GEM_DOMAIN_CPU) == 0) {
-		i915_gem_clflush_object(obj, false);
+		/* Invalidation not needed as there should not be any data in
+		 * CPU cache lines for this object, since clflush would have
+		 * happened when the object last moved out of CPU write domain.
+		 */
+		if (!obj->cache_clean)
+			i915_gem_clflush_object(obj, false);
+		obj->cache_clean = false;
 
 		obj->base.read_domains |= I915_GEM_DOMAIN_CPU;
 	}
-- 
1.9.2